The increasing traffic demand in optical networks appeals for the development of novel high spectral efficiency optical modulation schemes which would lead to the better use of the available optical bandwidth while increasing the operational bit rate. Increasing the spectral efficiency can be achieved by increasing the number of symbols in a signal constellation diagram via multilevel amplitude and/or phase encoding. Non-binary modulation schemes are a mature technology in wireline and wireless Radio Frequency based systems; their deployment into optical systems has been investigated for the last years.
Multilevel or M-ary amplitude shift keying (M-ASK) technologies, such as four level ASK (noted 4-ASK), offers better tolerance towards chromatic dispersion and Polarization Mode dispersion (PMD) compared to a conventional two level ASK technique (noted 2-ASK). Conventional methods to generate an optical 4-ASK signal are based on the modulation of a Continuous Wave (CW) light by an external modulator controlled by an electrical 4-ASK signal that is obtained by adding one binary signal to another (for example, intensity halved) binary signal.
For example, U.S. Pat. No. 7,110,681 describes a method and apparatus for optical transmission and is related to generation of a 4-ASK optical signal. The generation method is based on the transposition of a 4-ASK electrical signal into a 4-ASK optical signal. In such a configuration, moderate amplitude distortions of the electrical binary signals cause significant distortions of intermediate levels obtained in the resulting optical 4-ASK signal. Signal detection is performed by optoelectronic conversion followed by binary decoder using three reference voltage thresholds and a multilevel decoder.
All-optical methods for obtaining multilevel signals have been suggested to avoid the mentioned distortions.
The concept of all optical modulation is explained, for example, in U.S. Pat. No. 6,577,435. An optical wavelength converter, based on cross-gain modulation with wide input dynamic range, includes a semiconductor optical amplifier, a continuous wave source, and a probe beam controller. The semiconductor optical amplifier modulates probe power on the basis of pump power. The continuous wave source generates the probe beam and supplies the generated probe beam to the semiconductor optical amplifier. The probe beam controller adjusts bias current supplied to the continuous wave source and controls the probe power in proportion to the pump power.
U.S. Pat. No. 6,744,546 B2 describes an optical modulator integrated on a LiNBo3 substrate, optically combining one binary signal and another intensity halved binary signal.
Some all-optical techniques were proposed for optical 4-ASK generation.
For example, one technique is based on nonlinear effects in a semiconductor optical amplifier (SOA) [H. Soto et al. “All optical 2 to 4 level encoder based on cross polarization modulation in a SOA utilized to develop an all optical 2 input digital multiplexer” Optics Express, vol 14, n. 20 (2006)]. Another technique utilizes an electro-absorption modulation (EAM) [L. Huo et al, “Experimental; demonstration of a novel all optical multilevel 4-amplitude shifted keying coding/decoding scheme”, OFC 2006, paper JThB41 (2006)]. In both of these techniques, two independent 2-ASK (binary) modulated optical pumps encode a CW probe signal into an optical 4-ASK signal via the use of the cross gain modulation (XGM) and cross polarization modulation (XPo1M) in a SOA, or via the use of the absorption modulation (XAM) in an electro-absorption modulation (EAM) device.
US2005069330AA describes a system and a method for generating multilevel coded optical signals. The technique comprises driving modulators with synchronous data signals having the same data rate to generate a multilevel optical signal using a combination of differential phase shift keying (DPSK) and amplitude shift keying modulation.
A common feature of all optical multilevel modulation methods is the capability of providing all optical aggregation of the data carried at different optical channels, avoiding expensive and complex opto-electronics conversions.
Though, to the best of the Applicant's knowledge, all-optical systems for generating a multilevel coded optical signal by using the M-ASK technology are rather complex, since they comprise multiple signal generating schemes for producing component optical signals which are to be combined for obtaining the required multilevel optical signal. It is presently accepted that in such systems the number of signal-generating schemes linearly grows as log 2(M), where M is the number of required coding levels.